Demyelination is a significant contributor to functional loss after traumatic spinal cord injury (SCI). Demyelinated axons persist in the injured spinal cord chronically, and remyelination of demyelinated, but otherwise intact axons, represents an important repair strategy to facilitate functional recovery after SCI. Adult spinal cord has a limited capacity to spontaneously remyelinate, despite the fact that oligodendrocyte precursor cells (OPCs), which have the capacity to remyelinate, become active and proliferate in response to the injury. The differentiation of, and remyelination by endogenous OPCs is inhibited in the injured spinal cord. Transplantation of neural stem cells (NSCs) or glial progenitors has also proven effective to increase remyelination after SCI, but with varying degrees of success. The majority of grafted NSCs differentiate into astrocytes and their differentiation into oligodendrocytes (OLs) or neurons is inhibited in the injured spinal cord. Similarly, differentiation from grafted glial progenitors is also restricted in the injured spinal cord, and many transplanted glial progenitors remain undifferentiated or differentiate into astrocytes. Understanding the mechanism(s) that restrict the differentiation of and remyelination by the endogenous and grafted NSC or OPCs in the injured spinal cord should lead to new therapeutic strategies to repair SCI. The presence of inhibitory factors in the injury microenvironment may contribute to the restriction of the oligodendrocyte differentiation and remyelination. Our preliminary data showed that reactive astrocytes from the injured spinal cord inhibit OL differentiation of adult OPCs by increasing expression of bone morphogenetic proteins (BMPs). The absence of sufficient signals to stimulate differentiation of OPCs and myelination by mature OLs in the injured spinal cord may also contribute to the limited remyelination after SCI. Our preliminary data showed that increasing expression of growth factors such as the multi- neurotrophin D15A or CNTF promoted the remyelination by grafted OPCs in the injured spinal cord. We hypothesize that the combination of blocking inhibitory BMP signaling to promote OPC differentiation and increasing the expression of growth factors to enhance their maturation and remyelination may work synergistically to promote more extensive remyelination, and lead to greater electrophysiological and locomotor behavioral recovery after SCI. In this application, we will use objective and sensitive electrophysiological and behavioral analyses to test this hypothesis in a well characterized chemically demyelinated model and also clinically-relevant contusive SCI model in adult rats. Based on our published and preliminary data, four specific aims are proposed: 1. To determine whether astrocytes from the injured spinal cord inhibit differentiation of and remyelination by adult OPCs by increased secretion of BMPs. 2. To test the hypothesis that blocking inhibitory BMP signaling will promote the differentiation, maturation, and remyelination of grafted adult OPCs in the demyelinated spinal cord. 3. To test the hypothesis that combination of blocking inhibitory BMP signaling to promote the differentiation and increasing the expression of growth factors that enhance the maturation of and myelination by OLs will work synergistically to promote the differentiation, maturation, and remyelination of grafted adult OPCs and functional recovery in the demyelinated spinal cord. 4. To test whether the optimal combinatorial strategies, established in Aims 1-3, will lead to more extensive remyelination by transplanted OPCs and further functional recovery in the more clinically relevant contusion SCI model.